A known method for fabricating electrical circuits is to apply an adhesive solder paste to selected areas of a printed circuit board and then to place circuit components on the applied paste. The circuit board is heated to melt the solder paste. The melted solder wets metallic surfaces on the components and the circuit board. When the board is cooled the solder solidifies bonding the components to the circuit board and forming the electrical circuits.
Solder paste is applied through a stencil or screen with holes positioned to correspond to the areas where the components are to be connected with the circuit board. The circuit board is arranged below the stencil and aligned with the stencil holes. Solder paste is applied to the top surface of the stencil and a portion of the paste is forced through the stencil holes. This paste adheres to the circuit board. The circuit board is then removed from the stencil and replaced with a new circuit board and the process repeated. This process is most economically performed using automated machinery. For example, an automated apparatus for forming printed circuit boards using a stencil to apply solder paste is described in co-pending U.S. patent application Ser. No. 09/075,093, filed May 8, 1998.
In order to form reliable circuits using the screen printing technique it is critical that the stencil and the circuit board come in close contact during the printing process. Debris between the stencil and the board can cause solder paste to spread from its intended contact areas, resulting in short circuits. Further, debris within the holes of the stencil will prevent solder paste from flowing to the circuit board and will result in voids in the applied solder paste layer.
Debris is removed from the stencil by periodically wiping the bottom surface of the stencil with a fabric or paper sheet. In order to assure that the stencil surface is adequately cleaned a solvent is applied to the sheet.
FIG. 1 shows an apparatus 2 for wiping the bottom surface of a stencil 14. A cleaning sheet 1 is fed from a supply roll 3 and through a set of tensioning rollers 5. Solvent is applied to the sheet 1 by a solvent bar 11. The solvent is pumped into the solvent bar 11 by a pump (not shown) and sprays from the solvent bar 11 through jets 9 toward the sheet 1. The sheet 1 passes over a pair of flexible blades 13 which press the sheet 1 against the stencil 14. A vacuum channel 15 supports the blades 13. Suction is optionally applied to the vacuum channel 15 to pull the stencil 14 tightly against the sheet 1 as air is pumped from the channel 15 through an outlet tube 17. The sheet 1 then passes through a second set of tensioning rollers 19 and is taken up on a take-up roll 21. Cross members 16 are disposed across the width of the vacuum channel 15.
FIG. 2 shows a top view of the blades 13 and vacuum channel 15 shown in FIG. 1. The cross section of FIG. 1 is taken along line A in FIG. 2. A number of cross members 16 are disposed across the vacuum channel 15.
To wipe the stencil 14 the sheet 1 is advanced past the solvent bar 11 to apply solvent to the portion of the sheet 1 which will be positioned between the blades 13 and the stencil 14. The apparatus 2 is then moved as shown by arrows 23 so that the blades 13 wipe the solvent-laden sheet 1 across the stencil 14. Optionally, suction applied via the vacuum channel 15 increases the force between the sheet 1 and the stencil 14.
The DEK 256 GS Automatic Underscreen Cleaner, manufactured by DEK U.S.A., Inc., the assignee of the present invention, is an example of such an apparatus.
The use of solvent in the above apparatus facilitates the removal of debris, particularly dried solder paste. Without the use of the solvent, wiping using the above apparatus is unreliable. The use of solvents, however, presents a number of problems in the manufacturing process. Many solvents contribute to environmental pollution and are hazardous to workers. Certain solvent vapors can cause smog or contribute to ozone destruction in the upper atmosphere. Many solvents are toxic and/or highly flammable. As such, their use is heavily regulated and regulatory compliance increases the cost of manufacturing circuit boards. Less hazardous solvents are available but they are expensive and less effective in removing dried solder paste.
The apparatus described with reference to FIG. 1 can be used without applying a solvent to the sheet. The result, however, is that cleaning of the stencil is less effective and a greater number of unreliable solder joints are formed on the finished printed circuit board.
The amount of solvent that is required for the process can be reduced with acceptable reliability if workers periodically wipe the stencil by hand using a solvent soaked cloth instead of applying solvent to the automated wiping apparatus shown in FIG. 1. This procedure presents other difficulties, however. Hand wiping presents a health risk to workers by increasing their exposure to metals, particularly lead, in the solder paste. In addition, hand wiping requires that the automated machinery be stopped so that a worker can reach into the apparatus. Stopping the screen printer to wipe the stencil may result in idling equipment used in subsequent processing steps thus decreasing the productivity of the production line.